1. Field of the Invention
This invention relates to transceivers for transmitting and receiving electromagnetic radiation, and more particularly to transceivers for transmitting and receiving terahertz radiation.
2. Description of Related Art
A terahertz pulse can be produced by a device when a high speed optical pulse strikes a photoconductive switch generating electron-hole pairs in the semiconductor that causes the resulting charge carriers to flow between the photoconductive portion of a radiating antenna. This in turn emits an electromagnetic pulse from the antenna. The charge carrier population is quickly extinguished when the optical pulse is removed because of the fast carrier trapping speed that results from deep level traps within the semiconductor. This causes the ultrafast terahertz electromagnetic response to occur. Typical semiconductors used include low temperature grown gallium arsenide, low temperature grown indium gallium arsenide, and other suitable materials with the properties described. The semiconductor materials are typically designed with a direct band gap of the energy appropriate to absorb the incoming optical pulse efficiently.
The receiving antennas that detect the emitted terahertz electromagnetic radiation are often similar in construction and dimension to the transmitting antennas. The primary difference between the receiving antenna and the transmitting antenna is that the receiving antenna receives the incoming electromagnetic radiation which forms a small, but measureable, electric field at the antenna's photoconductive gap or switch. The imposed voltage bias resulting from this electric field is read by closing the photoconductive switch in the receiving antenna and measuring the induced current.
These terahertz systems usually use a pump-probe method of operation. Essentially, two antennas are used. The transmitting antenna is “pumped” with an optical pulse and emits the terahertz radiation. The receiving antenna is “probed” by a second pulse precisely time delayed from the first pulse. This time delay is often variable allowing for the sampling of the terahertz wave after it has been modified by a target object at different delay times from the initiation of the terahertz wave. The entire resulting waveform can be reconstructed by scanning the time delay of the probe pulse relative to the pump pulse.
Referring to FIG. 1, a prior art system 10 is shown of a known pump probe system. As its primary components, this system 10 includes a transmitter 12 for transmitting terahertz radiation 14 and a receiver 16 for receiving a portion 18 of the terahertz radiation 14 emitted by the transmitter 12. Examples of modules for transmitting and receiving terahertz radiation are disclosed in U.S. Pat. No. 6,816,647, which is herein incorporated by reference in its entirety.
Optical pulses used to excite the transmitter 12 and the receiver 16 are provided by optical fibers 20 and 22 which may be single mode optical fibers. A lens 24 directs terahertz radiation 26 towards a plate or sample 28. The plate or sample 28 reflects terahertz radiation 30, to a pellicle 32, which in turn reflects the reflected radiation 30 towards the receiver 16. These modules are fiber pigtailed and deliver short (10−14-10−12 second) optical pulses to the high-speed photoconductive switches. In the case of the transmitter 12, the short optical pulse activates a switch to generate a pulse of terahertz (1010-1013 Hz) radiation 26. This system uses a partially-reflective beam splitter, such as the pellicle 32, to overlap the beam paths of the transmitted and received terahertz beams.
One problem with this configuration is that approximately 75% of the terahertz power is lost when transmitted and returning signals encounter the pellicle 32. The transmitted signal loses half of its signal when initially encountering the pellicle 32. Half passes through the pellicle 32 to the plate or sample 28 being probed, while the other half is reflected away and lost. The return signal 30 encounters the same loss, as half is reflected by the pellicle 32 to the receiver 16, while the other half passes through the pellicle 32 and hits the transmitter 12 and is lost. Further, the configuration of the system 10 is also bulky, expensive, and difficult to align. It also requires that the fibers 20 and 22 be matched in length to deliver pulses to the transmitter 12 and the receiver 16. These fibers 20 and 22 can be problematic in that timing fluctuations caused by temperature changes, vibration effects, or simply stress imposed by twisting or pulling is imparted on one fiber more than it is the other fiber.